1. Field of the Invention
This invention relates to a method of determining grain size by measuring the attenuation of ultrasonic waves.
2. Description of the Prior Art
Examining how ultrasonic waves attenuate within materials offers an effective approach for determining their physical and other properties that are useful from the viewpoint of materials science. Especially, many attempts have been made at establishing relationships between the micro-structural characteristics of materials, such as the grain size of steel, and their attenuation constant of ultrasonic waves. When the steel structure consists of only one phase, ferrite or austenite, their grain size has been considered to be estimated. In reality, however, the steel structure is not limited to ferrite and austenite. There are such highly complex structures as pearlite, martensite, bainite and tempered martensite. Some of them are often even mixed in the same steel. Relationships of these structures with the attenuating characteristics of ultrasonic waves have not been very clear. It has been only vaguely guessed that different structures might have different attenuation constants. No definite relationship has been established between the attenuation constants and grain size.
The inventors already proposed a method of determining grain size using ultrasonic waves (refer to Japanese Patent Provisional Publication No. 126991 of 1988). However, this previous method had several limitations on measuring conditions: attenuation constant of ultrasonic waves .alpha.=2 dB/cm or above, discrimination accuracy =.+-.1.0 of ASTM grain-size numbers, and steel thickness=approximate 50 mm or under.